Etching process



Oct. 29, 1963 BOWMAN ETAL. 3,108,919

- ETCHING PROCESS Filed June 1'7, 1959 9 0 E 5 5 m a 28 0:0) 0 all. m [6 '65 o 0 2 2 0: LEI-Ll (DO. c

GRAM ATOMS CHROMIUM PER I000 GRAMS 0F SOLUTION INVENTORS DARL F. BOWMAN STEPHEN A. HAYS ATTORNEY United States Patent 3,108,919 ETCIWG PROtIESS Darl F. Bowman, Downey, and Stephen A. Hays, La lfillirada, Califl, assignors to North American Aviation,

Filed June 17, 1959, Ser. No. 820,909 18 Claims. (Cl. 156-18) This invention relates to a novel etching composition. More particularly, this invention relates to an etching composition suitable for etching metals which are soluble in hydrofluoric acid. This application is a continuationin-part of Serial No. 554,945, filed December 23, 1955, now forfeited.

Methods are known in the art for etching different metals. A problem presents itself, however, in the case of metals which are not readily reactive toward ordinary acids and bases as, for example, titanium, tantalum, zirconium, and alloys of these metals. Acids such as hydrogen fluoride and a mixture of hydrogen fluoride and nitric acid have been used for pickling and polishing such metals. However, these acids have been found to be inadequate for obtaining a smooth, evenly etched surface. A smooth, even surface is especially important when a metal is etched to produce a thin-walled metal structure with raised reinforcing areas to provide load distribution throughout the structure. The thin walls of such a structure should have a smooth and even surface in order that a predetermined maximum amount of metal may be removed from selected areas to produce [a panel of high strength-to-weight ratio. Etching mediums such as hydrogen fluoride alone or a combination of hydrogen fluoride and nitric acid, etc., do not produce the desired results. A need exists, therefore, for a better etching composition for use with metals which are dissolved by hydrogen fluoride.

An object of this invention, therefore, is to provide a novel etching composition.

Another object of this invention is to provide a composition which etches metal at a rate which can be readily controlled.

Still another object of the instant invention is to provided an etching composition which produces a smooth and even surface.

It is likewise an object of the present invention to provide an etching composition for etching metals which are dissolved by hydrogen fluoride.

Another object of this invention is to provide a process for etching metals, which are dissolved by hydrogen fluoride, at an even rate.

It is also an object of this invention to provide a process for etching metals wherein the components of the etching composition are maintained at a substantially constant concentration.

Another object of this invention is to provide a process for etching metals which results in a minimum of hydrogen absorption by the metal.

The above and other objects of this invention are accomplished by providing an etching composition comprising an aqueous solution of fluorine in the form of hydrogen fluoride and an amount of chromium in an anion form in amounts such that the proportions of fluorine and chromium present in the solution are defined by the area ABCDE of the FIGURE, the remainder being composed essentially of water. The use of this composition in the chemical milling of certain metals and metal alloys provides a process which results in controlled etching of surface to give a smooth surface etched to an even depth.

In the FIGURE the area ABCDE defines the relative proportions of fluorine and chromium in the solution in 3,l08,9l0 Patented Oct. 29, 1963 ice the form of anions. The anion form of the chromium is more fully described hereinbelo-w. The relative proportions of fluorine and chromium are given in terms of the number of gram atoms of the fluorine and the chromium per 1,000 grams of solution. For example, 1 gram atom of fluorine represents the grarnatornic weight of fluorine, namely, 19 grams. Hence, 1 gram atom of fluorine per 1,000 gnams of solution represents 19 grams of fluorine per 1,000 grams of solution. In like manner, 1 gram atom of chromium per 0,000 grams of solution represents 52.01 grams of chromium per 1,000 grams of solution. Other components may, of course, be present in the composition as more fully defined hereinbelow. Such other components include wetting agents such as, for example, sodium ethylbenzenesulfonate, and dissolved metal which usually is the result of the etching action of the aqueous hydrogen fluoride and chromium-containing anion solution on metals which are subjected to this etching bath. The area ABCD-E is obtained by connecting with straight lines the individual relative proportions represented by points A, B, C, D, and E. The relative proportions of these five points are shown in Table I.

TABLE I Point A B i 0 D l E Gram atoms Fluorine per 1,000 grams oisolution 12.5 12.5 3.5 3.5 6.5 Gram atoms Chromium per 1,000

grams oisolution 1.35 3.6 2 0.3 0.3

Area EFCD represents novel relative proportions of fluorine and chromium in the etching solution, the use of which provides a process which can be more readily controlled with respect to depth of etch and evenness of etched surface. The use of proportions as defined by the area EFCD is therefore preferred. The individual proportions of fluorine and chromium used to define area EFCD are shown in Table 11.

TABLE 11 Point E 1 F G D Gram atoms Fluorine per 1,000 grams of solution 6.5 6. 5 3. 5 3. 5 Gram atoms Chromium per 1,000 grams of solution 0. 3 2 2 0.3

TABLEIII I I I Point Gram atoms Fluorine per 1,000 grams of solution.; Gram atoms Chromium per 1,000 grams of solution O. 5

The examples and tables that follow indicate that the etching solutions which have been employed had fluoride ions and chromium-containing ions in amounts such that the atom ratio of fluorine-to-chromium is from about at this point.

an amount of HF in excess of 25 Weight percent be employed, a much greater amount of chromium-containing anion than that shown in the figure should be used in order to provide a composition which will dissolve metal at a controllable rate. At concentrations of fluoride ion above about 25 weight percent it is necessary to conduct the etching reaction at elevated temperature in order that a sufficient quantity of CrO may be dissolved to provide suflicient chromium-containing anion for a controllable etching rate. The chromium is added to the solution in the form of chromic anhydride, CrOg, or the alkali metal salts of chromic acid as, for example, sodium chromate, Na CrO sodium dichromate, Na Cr o etc.

Another embodiment of this invention comprises an aqueous etching solution, as outlined above, which in addition to the fluoride ion and chromium-containing ion, contains from 0.01 to 3 weight percent of a wetting agent such as a compound selected from the group consisting of a hydrocarbon derivative of sulfuric acid having an OM group directly attached to the sulfur atom, wherein represents oxygen, and M is selected from the group consisting of hydrogen and group I A alkali metals; and hydrocarbon glycol ethers; wherein the number of carbon atoms in said sulfuric acid derivatives and glycol ethers is from 6 to about 22. Examples of hydrocarbon derivatives of sulfuric acid are sodium ethylbenzene-sulfonate; potassium naphthylsulfonate; octylanthrylsulfonic acid; the sodium salt of l-sodiumcarboxylate-Z-N-octadecylcarboxamide ethylsulfonate,

CH (CONHC H CH COONa) SO Na;

the sodium salt of 1-ethyl-7-ethyl-4-undecylsulfate, etc. Thus, it is seen that the hydrocarbon derivatives of sulfuric acid can be either a sulfonic acid, a sulfate or a sulfonate; and also that there can be other substituents on the hydrocarbon portion of the molecule as, for ex- Nondimiting examples of glycol ethers are pheny ethyl ether of diethylene glycol, 1,2-diethoxyethane, dimenthylphenyl tetraethylene glycol ether, etc. When such hydrocarbon derivatives of sulfuric acid and/or glycol ethers are employed in our process, it is found that a much smoother surface is obtained than when these components are absent. The effect of these additives is particularly noticeable when etching a specimen of metal having only a portion of one surface exposed to the etching solution, the remainder of the surface being masked by a suitable resist material. As the etching solution dissolves the metal from the unmasked surface, the etching action is more severe at the edge of the masking, causing a groove to be etched in the exposed surface The addition of the above-mentioned sulfuric acid derivative and glycol ethers prevents the formation of this groove. Amounts of the additive in excess of the range specified above should not be used, as they have the effect of causing a ridge to be left in the etched surface just inside the edge of the masked port-ion. Compounds other than the above, such as, for example, octadecyl-amine and guanidine salts of octadecyl carbonic acid, had no efiect on the evenness of the etched surface, illustrating the uniqueness of the sulfuric acid derivatives and glycol ethers used in this invention.

One of the preferred embodiments of this invention is the use of the hydrocarbon derivatives of sulfuric acid and/or hydrocarbon glycol ethers in amounts of from having atomic weights of from 28 to 181, as well as for low alloys of these metals; and the process of etching such metals with our composition is another embodiment of this invention. Reference is made to page 35, Hackhs Chemical Dictionary, by Grant, 3rd edition, 1944 for a definition of the term low alloy, which is an alloy of the large constituent metal. The periodic table of elements referred to in this writing is that found in the Handbook of Chemistry and Physics, pages 392-3, 37th edition, 1955-1956, published by the Chemical Rubber Publishing Company, Cleveland, Ohio. The various titanium alloys, for example, are readily etched by the use of the process of this invention providing a smoothly etched surface, etched to an even predetermined depth. The titanium alloys can contain one or more of the elements vanadium, aluminum, molybdenum, iron, chromium, and manganese, as well as other metallic elements,

in varying amounts. Non-limiting examples of titanium alloys etched by the instant process are given in the specific examples and tables hereinbelow. To illustrate the process of this invention titanium is etched by a method comprising contacting the metal with an etching composition consisting of a solution containing 12 weight percent hydrogen fluoride, 10 weight percent chromic anhydride, 0.1 percent of dodecylbenzene sulfonic acid, and the remainder consisting of essentially water.

A preferred embodiment of this invention is the process for etching a substance selected from the group consisting of the elements, Si, Ge, group lV-B and group .V-B elements of the periodic table of elements, having atomic weights of from 28 to 181 and alloys of said elements, comprising contacting said metal with an aqueous solution of from 7 to 25 weight percent hydrogen fluoride and an amount of a chromium-containing anion, which can also be designated an oxygenated chromium anion,

such that the relative amounts of fiuorine-tochromium in terms of gram atoms per 1,000 grams of solution is defined by the area ABCDE of the figure, and adding potassium fluoride to the etching solution during the operation of the process in an amount such that the molar ratio of KF added-to-metal in solution is from about 1:5 to

about 2:1, to precipitate the metal dissolved during the especially preferred to add an amount of potassium fluoride such that the molar ratio of KF-to-Ti in solution is Amounts of KF sufficient to thoroughly precipitate all of the titanium in the solution can be added. However, it is preferred not to add an excess of KF in order to avoid any deleterious effects that the potassium ion may have on the etching process. A convenient method for precipitating the titanium from the solution is to withdraw a portion of the solution from the etching container, treat it with KF, filter out the precipitated titanium compound which is usumly K TiF and return the filtrate to the etching container. In this manner the titanium can be continuously removed from the etching solution with recycle of the filtrate. It is unexpected that the addition of KF should preferentially remove dissolved Ti from a solution also containing CrF which is probably formed in the etching solution according to the over-all equation since it is known that KF reacts with CrF to give the insoluble product K CrF From the above equation, it is seen that chromium fluoride is another component that is present in the solution as the etching of a metal or of its alloy proceeds. The CrF is precipitated from the etching solution as the concentration of the chromic fluoride reaches saturation proportions, and therefore it is not necessary to add a precipitant for this compound. Other alkali metal fluorides may also be used in place of KF. However, when NaF is employed, all the CrF is precipitated first as Na CrF This requires a greater amount of reagent than is the case when Kl is used, and is an unnecessary waste, since CrF can be precipitated from a saturated solution. The potassium fluoride method of precipitating dissolved titanium has the added advantage that it provides a method for separating chromium from titanium. A further advantage in the use of potassium fluoride is that a maximum amount of titanium is precipitated with a minimum amount of potassium ion being left in solution, thus minimizing any detrimental effects that alkali metal ions may have on the process. Hydrogen fluoride can be continuously added to the etching solution to replace that used up in the process. The steady rate thus attained results in a more even etching of the specimen as well as a more readily controlled process. In addition, an economic advantage is secured due to recycle or reuse of the etchant.

The use of the novel etching composition of this invention not only provides means for controlling the rate of metal removal, and evenness of the etched surface, but also results in a minimum of hydrogen absorption in the metal. The hydrogen is formed in the attack of metal by the etchant. In the case of titanium this can be expressed by the equation The absorbed hydrogen problem is particularly important in metals such as titanium which absorb hydrogen readily causing embrittlement and consequently loss of structural strength. Thus, the use of the etching compositions in the process of our invention in a manner taught herein provides a process for etching metals of the kind hereinabove specified to produce a metal specimen with a minimum of absorbed hydrogen.

As stated above, the rate of etching by a composition of this invention can be readily controlled. This is a decided advantage over compositions such as an aqueous solution of HF alone, or a combination of HF and HNO For example, an aqueous solution of 12 weight percent of HF etches titanium at a rate of 70 mils per hour at 110 F., and at a rate of 100 mils per hour at 120 F. One mil, as used in this writing, represents one-thousandth of an inch. The reaction is exothermic, heating up the solution. This rapidly increases the rate which is a function of the temperature, making it difficult to keep the reaction from getting out of control. An even greater increase in rate is observed with an aqueous solution containing 7.5 weight percent HF and 12.5 weight percent HNO which etches titanium metal at a rate of 40 mils per hour at 110 F, and increases to a rate of 200 mils per hour at 120 P. On the other hand, a combination of HF and CrO in proportions of 12 and weight percent in water, respectively, gives an etching rate of 42 mils per hour at 110 =F., and 52 mils per hour at 120 F. This is only one-third of the increase in rate noted with an aqueous solution of HF alone, due to a 10 F. rise in temperature, and but one-sixteenth of the increase noted with an HP HNO mixture, thus making the reaction easier to control.

The above and other objects of this invention are more clearly illustrated by the :following examples in which the novel etching composition is used to remove predetermined amounts of metal from different metal objects. The proportions are given in parts by weight unless otherwise specified.

Example I To a container coated with an unplasticized polyvinyl chloride composition which is resistant to acid attack and equipped !with heating and cooling means, and means for agitation, there were added 780 parts of water, parts of chromio mhydride and parts of hydrogen fluoride. This composition represents a solution containing 78 weight percent water, 12 weight percent HF and 10 weight percent CrO based on the total weight of the mixture. There were 6 gram atoms of fluorine and 1 gram atom of Cr per 1000 grams of composition as represented by point 1 in the figure. The F:Cr atom ratio was 6:1. The contents of the reaction vessel were then thoroughly mixed by the agitating means until the solution was of a homogeneous composition. The temperature of the mixture was brought up to substantially F. by the heating means and then a specimen of titanium alloy containing five percent manganese, having dimensions 2" x 4" x 0.125, was immersed and held in a vertical position by means of vinyl plastic covered wires. The etching composition was allowed to act on the titanium specimen for a period of 20 minutes, at the end of which time the titanium was removed from the etching composition and washed with water. An amount of titanium equivalent to a thickness of 59 mils was found to have been removed leaving an even and smooth finish.

The etching rate was thus 177 mils per hour.

Example 11 Following the procedure of Example I, an etching composition is made up containing 872 parts of water, 70 parts of hydrogen fluoride, 58 parts of chromic anhydride, and 0.1 part of hexane sulfonic acid. This composition represents 7 weight percent HF, 5.8 weight percent CrO and 0.01 weight percent hexane sulfonic acid based on the total weight of the solution. There are 3.5 gram atoms of fluorine and 0.58 gram atoms of chromium per 1000 grams of solution as represented by point 2, in the figure. The F:Cr atom ratio is 6:1. A specimen of vanadium measuring 3" x 5" x 0.2" and masked with a vinyl plastic composition on one face, as Well as the edges, and having /4 inch masked border on the exposed surface, is immersed in the solution for a period of 20 hours at a temperature of 30 F. Upon removal from the solution, the specimen is found to have an evenly etched smooth surface.

Similar results are obtained when silicon is etched according to the procedure of Example II.

Example III The procedure of Example I is repeated with a zirconium specimen using an etching composition containing 854 parts of water, 80 parts of HF, 66 parts of CrO and 0.8 part of sodium dodecylnaphthyl sulfonate representing a solution containing 8 weight percent HF, 6.6 weight percent CrO and 0.08 weight percent of the sulfonate. The mixture contains 4 grams atoms of fluorine and 0.66 gram atoms of chromium per 1000 grams as represented by point 3 in the figure. The atom ratio FzCr is about 6:1. The etching is carried out at a temperature of 210 -F., and after five minutes, the exposed surface of the Zirconium metal is found to be evenly etched to a uniform depth.

Equally good results are obtained when Example III is repeated with the dodecylnaphthyl sulfonate replaced by sodium Z-ethylhexyl sulfate: the sodium salt of l-sodiumcarboxylate 2 N -octadecylcarbona1mide ethylsulfonate CH (CONHC H -,)CH(COONa)SO N a; and the sodium salt of 1-methyl-7-ethyl-4-undecylsulfate.

Example IV The procedure of Example I was repeated using an etching composition containing 780 parts of water, 120 parts HF, 100 parts CrO and 1 part of dodecylbenzene sulfonic acid. There were 6 gramatoms of fluorine and 1 gram atom of chromium per 1000 grams of solution as represented by point 1 in the figure. A specimen of titanium masked with lead-backed tape on one surface, on the edges, and having a 4 inch lead-backed tape border on the exposed surface was suspended in a horizontal position and subjected to the action of the etching com position at a temperature of substantially 155 F. for a period of 30 minutes, after which time the specimen was removed and thoroughly washed. The specimen was found to be etched to a depth of 62 mils, thus indicating the rate of etching to be 124 mils per hour. The surface was smooth and even throughout.

Example V The procedure of Example I was repeated using 620 parts of water, 180 parts of HF, 200 parts of CrO and Example VI Following the procedure of Example I, a specimen of titanium prepared as in Example IV, was suspended horizontally in an etching composition consisting of 765 parts of water, 120 parts of HF, 50 parts CrO 65 parts of Na Cr O and 1 part of dodecylbenzene sulfonic acid. The F :Cr atom ratio in this composition was 6:1 and the composition is represented by point 1 in the figure with regard to the fluorine and chromium concentrations. The titanium specimen was subjected to the action of the etching solution for a period of 1 /2 hours at a temperature of 150 F. tion etched the titanium at the rate of 65 mils per hour, producing an excellent finish.

Example VII The procedure of Example VI was repeated, except that the temperature was kept at 110 F. In this case, it was found that the etching proceeded at a rate of 43 mils per hour, and resulted in an even, wrinkle-free surface.

Example VIII The procedure of Example I is followed, using 633 parts of water, 200 parts of HF, 167 parts or CrO' and 1 part of ethyl pheny-l ether of diethylene glycol, at a temperature of 130 F., to obtain a smoothly etched surface at a controllable rate. There are atoms of fluorine and 1.67 atoms of chromium per 1000 grams of solution as represented by point 8 in the figure. The F :Cr atom ratio is 6: 1.

Equally good results are obtained when Example VIII is repeated with the phenyl ethyl ether of diethylene glycol replaced by 1,2-diethoxy ethane, 1-ethoxy-2-isopropoxy-3- ethylphenoxy propane, and di-methylphenyl tetraethylene glycol ether.

Example IX The procedure of Example VIII is employed with a composition containing 20 weight percent HF and 28.6 weight percent CrO This composition contains 10 gram atoms of fluorine and 2.86 gram atoms of chromium as It was found that during this time, the solu- 8 represented by point 9 in the figure. The F:Cr atom ratio is 3.5:1.

ExampleX In this case, the procedure of Example I was followed, employing a solution consisting of 730 parts of water, parts of HF and parts of CrO' There were 6 gram atoms of fluorine and 1.5 gram atoms of chromium per 1000 grams of solution as represented by point 10 in the figure. The titanium specimen was subjected to the action of the solution for a period of 1 /2 hours at a temperature of 190 F. The rate of etching was checked periodically and was found to vary from 42 mils per hour at the beginning of the etching period, to 12 mils per hour after the first /2 hour. The FzOr atom ratio was 4:1 in this case.

Example XI Following the procedure of Example X, titanium is etched with a solution containing 25 Weight percent HF, and 30 weight percent CrO such that the HF:CrO molar ratio is 4.2.11.0. There are 12.5 gram atoms of fluorine and 3 gram atoms of fluorine per 1000 grams of solution as represented by point 11 in the figure.

Example XII container. In this manner, the amount of titanium in solution was maintained at a concentration such as not to adversely atfect the rate of etching.

Example XIII Example XII is repeated, adding KF at 80 F. in an amount such that the ratio of .KF ad ded-tmTi in solution is 1:5, giving good results.

Equally good results are obtained when the ratio of KF added-to-Ti in solution is 2:1, the addition of KF being made 'at F.

Good results are also obtained when the concentration of the titanium in the etching solution is 0.1 mol per liter, and likewise when the concentration of Ti is 3 mols per liter and higher. A preferred range of Ti concentration in the etchant is up to about 80 grams per liter, in order.

to maintain a more practical rate and more even etching.

Similar results are obtained when the procedure of Example XII is followed in the etching of substances of Si, Ge and the metals of group IV-B and group V-B of the periodic table of elements as well as alloys of these elements, when the concentration of said substance in solution is from 0.1 to 3 molar, and the amount of alkali metal fluoride added is such that the ratio of the fluoride added-to-metal in solution is from 1:5 to 221 at temperatures of from 80 F. to 160 F. Preferably the amount of said elements and said alloys in solution is from about 0.1 grant to about 90 grams per 1000 grams of solution as this results in a more readily controllable process.

Example XIV The procedure of Example I is followed employing the etching composition described therein in etching a specimen of germanium. The specimen is kept in the solution for a period of time suflicient to remove an amount of germanium equivalent to a thickness of 30 mils from the surface. The etched surface is smooth and etched to an even depth.

Example XV The procedure of Example XIV is followed in the TABLE IV TITANIUM CONTAINING 16 WT. PERCENT V AND 2.5 WT. PERCENT A1 Cram Gram Repre- Atoms Atoms Atom sented Example No. F per Cr per Ratio, in Fig- 1,000 g 1,000 g. FzCr ure by solution solution Point 0.5 10.1 12 6.1 05 12521 G 5 1.35 3.7:1 13 6.1 1.35 4.521 H 6.5 0.3 21.7:1 E

TITANIUM CONTAINING 4 WT. PERCENT A1, 3 WT. PER- CENT Mo, AND 1 WT. PERCENT V 4 0. 5 8:1 I 6.1 0.5 12.5:1 G 4 1.35 2. 96:1 I 6.1 1. 35 4. 5:1 H 3.5 0.3 11.7:1 D

TITANIUM CONTAINING 7 WT. PERCENT A1 AND 3 IT. PERCENT IVIO TITANIUM CONTAINING 4 WT. PERCENT A1, 1.5 WT. PER- CENT Fe, 1.4 WT. PERCENT Cr, AND 1.2 WT. PERCENT Mo In the use of the etching compositions on the specimens indicated in Table IV above, the amount of metal in solution varied from 0 to the saturation point. In all cases good results were obtained as to the smoothness of surface and evenness of depth of the etched portion, however, it is found that when the metal concentration approaches the saturation point, the process becomes more ditficult to control. However, Example 2. in Table IV containing about 90 grams of the alloy being etched in solution per 1000 grams of etching bath, gave a good product. Good results are obtained when the metal concentration is substantially 8 grams per 1,000 grams of solution in the case of Example 14 in Table IV. Likewise, good results are obtained with a metal concentration of substantially 40 grams per 1,000 grams of solution in the case of Example 6 in Table IV. Hence, a preferred embodiment of this invention is the process of this invention carried out with the solution containing from about 8 to about 40 grams of metal in solution per 1,000 grams of etching composition or bath. In general, the process of this invention is one in which the elements or alloy in question is subject to the etching action of the compositions described hereinabove which contain said elements or alloys in solution in an amount of from 0% to the amount required for saturation.

Smooth surfaces etched to an even depth over the surface exposed to the etching bath were obtained in etching the specimen listed in Table IV with the compositions indicated. The etch rate for the compositions in Table IV was found to vary from about 0.5 to about 1.0 mil per surface exposed to the etching bat'h per minute.

The metal specimens which are subjected to the action of the etch-ant may be suspended in the solution either in a horizontal or vertical position, as indicated by the examples. The rate of etching is faster when the specimen is suspended vertically as compared to the rate of horizontal suspension. In both cases, however, the etching proceeds to give a smooth, even fim'slh.

The masking in the examples above consisted of leadbacked tape and vinyl plastic coating; however, any other means of coating the surfaces which are not to be etched are also used in practicing our invention. For example, polyethylene on cotton tape with silicon rubber adhesive is used as a masking substance. Likewise, neoprene rubber paint is a good resist for covering surfaces which are to be protected from the action of the etching solution.

The length of time for which the metal is subjected to the action of the solution depends on the concentration of hydrogen fluoride and chromic oxide in this solution, as well as the depth to which the exposed surfaces are to be etched. Thus, metal specimens can be exposed to the novel etching composition of this invention for periods ranging from a matter of five minutes to as high as 20 hours or longer.

Compounds which yield HF in situ may be used toreplace HF as, for example HBF H TiF and H SiF etc. In such instances, however, it usually is necessary to raise the reaction temperature to obtain satisfactory etching. The life of the etching solution is also found to be much lower.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

We claim:

1. A process for etching a substance selected from the group consisting of the elements of silicon, germanium, group IV-B elements and group V-B elements of the periodic table of elements, and low alloys of said elements, comprising contacting said substance with an etching composition comprising an aqueous solution of hydrogen fluoride and an oxygenated chromium anion wherein said hydrogen fluoride and said oxygenated chromium anion are present in amounts such that the relative proportions of fluorine and chromium are defined by the area ABCDE of the figure.

2. The process of claim 1 wherein the relative proportions of fluorine and chromium are defined by the area EFCD of the figure.

3. The process of claim 1 wherein the relative proportions of fluorine and chromium are defined by the area GHU of the figure.

4., A process for etching a substance selected from the group consisting of the elements of silicon, germanium, group IV-B elements and group V-B elements of the periodic table of elements, and alloys of said elements, comprising contacting said substance with an etching composition comprising an aqueous solution of hydrogen flu-o ride and an oxygenated chromium anion wherein said hydrogen fluoride and :said oxygenated chromium anion are present in amounts such that the relative proportions of fluorine and chromium are defined by the area ABCDE of the figure, and from about 0.01 to about 3 weight percent of a compound selected from the group consisting of (1) a hydrocarbon derivative of sulfuric acid having an OM group directly attached to the S atom, wherein 0 represents oxygen and M is selected from the group con- 1 l sisting of hydrogen and the groupl A alkali metals, and (2) a hydrocarbon glycol ether, wherein the number of carbon atoms in said sulfuric acid derivative and glycol ethers is from about 6 to about 22.

5. The process of claim 4 wherein the relative proportions of fluorine and chromium are defined by the area EFCD of the figure.

6. The process of claim 4 wherein the relative proportions of fluorine and chromium are defined by the area GHII of the figure.

7. Aprooess for etching titanium metal comprising contacting said metal with an etching composition comprising an aqueous solution of hydrogen fluoride and an oxygenated chromium anion wherein said hydrogen fluoride and said oxygenated chromium anion are present in amounts such that the relative proportions of fluorine and chromium are defined by the area ABCDE of the figure, and from about 0.01 to about 3 weight percent of a compound selected from the group consisting of (l) a hydrocarbon derivative of sulfuric acid having an OM group directly attached to the S atom, wherein represents oxygen and M is selected from the group consisting of hydrogen and the group I-A alkali metals, and (2) a hydrocarbon glycol ether, wherein the number of carbon atoms in said sulfuric acid derivative and glycol ethers is from about 6 to about 22.

8. A process for etching a titanium alloy comprising contacting said metal with an etching composition com prising an aqueous solution of hydrogen fluoride and an oxygenated chromium anion wherein said hydrogen fluoride and said oxygenated. chromium anion are present in amounts such that the relative proportions of fluorine and chromium are defined by the area ABCDE of the figure and from about 0.01 to 3 weight percent of a compound selected from the group consisting of (1) a hydrocarbon derivative of sulfuric acid having an OM group directly attached to the S atom, wherein 0 represents oxygen and M is selected from the group consisting of hydrogen and the group I-A alkali metals, and (2) a hydrocarbon glycol ether, wherein the number of carbon atoms in said sulfuric acid derivative and glycol ethers is from about 6 to about 22.

9. The process of claim 8 wherein the relative proportions of fluorine and chromium are defined by the area EFCD of the figure.

10. The process of claim 8 wherein the relative proportions of fluorine and chromium are defined by the area GHIJ of the figure.

11. A process for etching a titanium alloy comprising contacting said alloy with an etching composition comprising an aqueous solution of hydrogen fluoride and chromic anhyd-ride in amounts such that there are substantially 6 gram atoms of fluorine and 1 gram atom of chromium per 1,000 grams of said solution, and substantially 0.1 weight percent :of dodecylbenzene sulfonic acid.

12. A process for etching a substance selected from the group consisting of the elements of silicon, germanium, group -IVB elements and group V-B- elements of the periodic table of elements, and low alloys of said elements comprising contacting said substance with an etching composition comprising an aqueous solution of hydrogen flu oride and an oxygenated chromium anion in the form of at least one component selected-from the class consisting of chromic anhydride and the alkali metal salts of chromic acid, wherein the hydrogen fluoride and oxygenated chromium anion are present in amounts such that the relative proportions of fluorine and chromium are defined by the area ABCDE of the figure.

'13. A continuous process :for etching a substance selected from the class consisting of the elements of silicon, germanium, group IV-B elements and group V-B elements of the periodic table of elements, and alloys of said elements in a manner such that the concentration of dissolved metal is maintained below a point which would adversely affect the rate of etching and evenness of etched surface comprising contacting said substance with an etching composition comprising an aqueous solution of hydrogen fluoride and an oxygenated chromium anion 'wherein said hydrogen fluoride and said oxygenated chromium anion are present in amounts such that the relative proportions of fluorine and chromium are defined by the area ABCDE of the figure; allowing the process to operate until the amount of metal dissolved in said solution .reaches a value of from about 0.1 mol per liter of said solution to about the saturation point of said solution; withdrawing a portion of said solution from the etching container; adding an alkali metal fluoride thereto in an amount such that the molar ratio of said alkali metal fluoride addedto-the-l metal in solution is from about 1:5 to about 2:1 to precipitate said metal, filtering out said precipitate and returning the filtrate to the etching container.

14. The process of claim 13 wherein the said alkali metal fluoride is potassium fluoride.

15. The process of claim 13 wherein said substance being etched is an alloy of titanium and said alkali metal fluoride is potassium fluoride.

16. A continuous process for etching a substance selected from the group consisting of the elements of silicon, germanium, group 'IV-B elements and group V-B elements of the periodic table of elements, and alloys of said elements in a manner such that the concentration of dissolved metal is maintained below a point which would adversely affect the rate of etching and evenness of etched surface, comprising contacting said substance with an etching composition comprising an aqueous solution of hydrogen fluoride and an oxygenated chromium anion wherein said hydrogen fluoride and said oxygenated chroarea ABCDE of the figure and from about 0.01 to about 3 weight percent of a compound selected from the group consisting of (1) a hydrocarbon derivative of sulfuric acid having an OM group directly attached to the S atom, wherein 0 represents oxygen and M is selected from the group consisting of hydrogen and the group I-A alkali metals, and (2) a hydrocarbon glycol ether, wherein the number of carbon atoms in said sulfuric acid derivative and glycol others is from about 6 to about 22, allowing the process to operate until the amount of metal dissolved in said solution reaches a value of from 0.1 ,to about 3 mols per liter of said solution; withdrawing a portion of said solution from the etching container; adding an alkali metal fluoride thereto in an amount such that the molar ratio of said alkali metal fluoride added-to-t-he-l metal in solution is from about 1:5 to about 2:1 to precipitate said metal, filtering out said precipitate and returning the filtrate to the etching container.

17. The process of claim 16 wherein said alkali metal fluoride is potassium fluoride.

18. The process of claim 16 wherein said substance being etched is an alloy of titanium and said alkali metal fluoride is potassium fluoride.

References Cited in the file of this patent UNITED STATES PATENTS 1,836,445 Chapman Dec. 15, 1931 2,172,041 Urban Sept. 5, 1939 2,378,052 Waldman June 12, 1945 2,620,265 Hesch Dec. 2, 1952 2,698,781 Meyer Jan. 4, 1955 2,828,193 Newman Mar. 25, 1958 2,847,287 L'andgren Aug. 12, 1958 2,867,514 Newhard et al Jan. 6, 1959 2,871,110 Stead Jan. 27, 1959 2,886,420 Jones etal. May 12, 1959 2,904,413 Hampel Sept. 15, 1959 

1. A PROCESS FOR ETCHING A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF THE ELEMENTS OF SILICON, GERMANIUM, GROUP IV-B ELEMENTS AND GROUP V-B ELEMENTS OF THE PERIODIC TABLE OF ELEMENTS, AND LOW ALLOYS OF SAID ELEMENTS, COMPRISING CONTACTING SAID SUBSTANCE WITH AN ETCHING COMPOSITION COMPRISING AN AQUEOUS SOLUTION OF HYDROGEN 